What is the word to describe a person who popularized a sport? The person does not need to be one of the original players nor even currently active. An example is Arnold Schwarzenegger popularizing competitive bodybuilding. There were many superstar bodybuilders before him, like Eugen Sandow, and he has been inactive for a decade. Arnold is, no doubt, the reason why many people started bodybuilding.

The word 'champion' might be used in the sense of an advocate or proponent, however in the context of sport the use of 'champion' may be ambiguous. There's always 'popularizer'...
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SnubianMay 3 '11 at 5:42

One problem with this idiom: heroes come and go, and new heroes take their place (particularly in competitive athletics). Bill Russell, John Havlicek, Larry Bird, and Paul Pierce have all had their turns as the face of the Boston Celtics. So, for the purposes of your question, you could adapt this phrase, and say:

Babe Ruth was the original face of the Yankees.
Red Auerbach was the original face of the Celtics.
Vince Lombardi was the original face of American football.
Arnold Schwarzenegger was the original face of competitive bodybuilding.